Contents

Introduction

This document describes how to map out the multi-destination tree for a given Forwarding Tag (FTag) in a FabricPath topology. This allows you to follow the expected flow of a multi-destination packet for a given FTag. In this example, you start from the FabricPath edge switch N7K5-FPE and you map out the FTag 1 tree. The full FabricPath domain topology is shown in the diagram.

Prerequisites

Users should be familiar with FabricPath concepts and terminology in order to use this document.

Requirements

There are no specific requirements for this document.

Components Used

The information in this document is based on these software and hardware versions:

Nexus 7000 with Release 6.1(2)

F2 series line cards.

The information in this document was created from the devices in a specific lab environment. All of the devices used in this document started with a cleared (default) configuration. If your network is live, make sure that you understand the potential impact of any command.

Background Information

Users should be familiar with FabricPath concepts and terminology. A brief explanation of the use of the FTag ( forwarding tag) parameter in the FabricPath header is covered in this section.

The function of FTag is detected by the type of frame. Specifically, if the frame is a unicast of multi-destination. In the case of a unicast frame, the FTag identifies and selects the FabricPath Topology for which the given frame should traverse. Single topology is supported with a value of "1" assigned to it.

In the case of multi-destination frames, the ingress switch that receives the frame, needs to identify which multi-destination forwarding tree the given frame traverses.

When multi-destination traffic enters a FabricPath domain, the ingress switch uses a hashing algorithm in order to decide which FTag to program in the FabricPath header. Each FabricPath Topology has two multi-destination trees, FTag 1 and FTag 2 trees. Each FTag has a root switch that is calculated similar to a Spanning-tree root. The election is based on FabricPath priority and system ID. The switch with the highest priority, or the system ID when the priority is defaulted, becomes the root for FTag 1 and the runner-up is the root for FTag 2.

Once a FTag is selected by the ingress FabricPath edge switch, the rest of the FabricPath core forwards the multi-destination packet based on that FTag. A multi-destination packet includes any broadcast, multicast, or unknown unicast packet. Each switch forwards the packet based on the lowest cost to the root. Once the root receives the packet, it forwards it to all switches in that FTag except the switch from which it was received.

Map Out the Multi-destination Tree for a FTag

Confirm the local switch-ID.

Note: When a FabricPath switch is a member of a vPC+ domain, it has a non-emulated (standalone) switch-id and an emulated (vPC+) switch-id. In the output example, note that this system-ID (6c9c.ed4f.28c4 ) is shown twice. Once for the non-emulated switch-id, and once for the emulated switch-id.

Check the local switch-id in order to determine if you are the root, or if you need to move towards the root. The output example shows that this system is switch-id 21. You know this from Step 2 and Step 6. It is the root for FTag 1.

Since you know N7K5-SPINE3 is the root, you need to see how it forwards a multi-destination frame received with FTag 1. Based on this output, N7K5-SPINE3 forwards a multi-destination frame with FTag 1 to Eth6/5 - Eth6/8 and Port-channel 1.

N7K5-SPINE3# show fabricpath isis trees multidestination 1Fabricpath IS-IS domain: defaultNote: The metric mentioned for multidestination tree is from the root of that tree to that switch-id